A number of circuit breaking devices are presently employed for protecting electrical circuits against current overloads. The fuse is the most widely used and least expensive of these devices. However, a short circuit or overload will blow the fuse and thus render it unusable. Replacement of the fuse is required after each operation. Alternatively, reusable heat-sensitive elements, such as bimetallic strips or thermistors, have been utilized as circuit breakers. Bimetallic strips bend upon overheating to directly open a current-breaking switch. Continuously acting bimetallic strips are typically used only in motors as a supplement to line protection fuses and magnetic circuit breakers. Such strips are able to withstand starting current surges without breaking, but are much too slow in operation to entirely replace the fuses or magnetic breakers. Bimetallic strips of the snap-over variety are often used as thermostats in irons, toasters, and other appliances. Such circuit breakers act rapidly but the temperature at which they break the circuit is typically imprecise. Thermistors are composed of ceramics which change their resistance abruptly at predetermined temperatures. Although useful as overload sensors, thermistors react too slowly to temperature changes to completely handle circuit breaking functions. In magnetic circuit breakers, an overload current activates an electromagnet which in turn opens a circuit breaking switch. Such devices are fast-acting. However, they are also relatively expensive and require a current surge for activation.